Hydrogels are candidate biomaterials for device applications such as synthetic articular cartilage replacement in humans and animals. Hydrogels are polymeric structures that are hydrophilic and contain high concentrations of water. Typically hydrogels have the desirable properties of biocompatibility, low coefficient of friction, and high water content. However, most hydrogels lack the mechanical properties necessary to sustain the high loads that are present in most of the joints in the body. For instance, the axial load in the knee can go as high as three to five times the body weight of the patient. Under such loads, a hydrogel material replacing the function of the articular cartilage is expected to maintain its shape and function in the long-term. Some of the expected requirements from a hydrogel material to resist the mechanical cyclic loading and articulation of human joints are high stiffness, high strength, high toughness, high wear resistance, and/or low coefficient of friction against an opposing cartilage surface.
Most hydrogels systems available for articular cartilage replacement applications do not have required mechanical strength to withstand the high loads of the human joint. Various dehydration and deformation methods, described below, can be used together in combinations to improve the properties of hydrogels.
Solvent dehydration of hydrogels is described by Bao (U.S. Pat. No. 5,705,780). Bao describes immersion of PVA hydrogel into solvents such as ethanol/water mixture at room temperature to dehydrate PVA hydrogel without shape distortion.
Hyon and Ikada (U.S. Pat. No. 4,663,358) and Bao (U.S. Pat. No. 5,705,780) describe the use of water and organic solvent mixture to dissolve PVA powder and subsequently cooling the solution below room temperature and heating back up to room temperature to form a hydrogel. The hydrogel is then immersed in water to remove the organic solvent. Hyon and Ikada claim that PVA hydrogels thus formed are transparent as opposed to the ones formed by freeze-thaw method that uses water only as the solvent to dissolve the PVA powder.
Bao (U.S. Pat. No. 5,522,898) describes dehydration methods that use air dehydration, vacuum dehydration, or partial humidity dehydration to control the rate of dehydration and prevent shape distortion of PVA hydrogels for use as prosthetic spinal devices to replace the nucleus pulposus. The starting gels of Bao are the freeze-thaw gels described in the U.S. Pat. No. 5,705,780.
Ku et al. (U.S. Pat. No. 5,981,826) describes a freeze-thaw method to form a PVA hydrogel by subjecting a PVA aqueous solution to freeze thaw followed by immersion in water and additional cycles of freeze-thaw while immersed in water.
However, until the instant invention, the field lacked tough hydrogels for articular cartilage replacement applications that have required mechanical strength, and can withstand high loads of human joints.